Holley Mods

I did it on a manual mill in a city college machine shop night class. We also practiced on a junk carb using a hacksaw to cut off the horn, then used a die grinder for final finish work. Other than the milled finish the results were identical.

I did mine but I would imagine any competent shop with an end mill could do it.

Milling the air horn is done to increase flow, BUT there are some bad sides to it in some situations when the carb is exposed to open air (no air cleaner at high speeds).

Milling the choke horn is no big deal. We use a mill but you can easily do it with a simple die grinder and a carbide burr. Even when we mill them, we still go in and contour it with a die grinder, then hit it with a sanding roll to smooth out and remove any sharp spots on the edges.

Contrary to popular belief, milling the air horn won't do anything for a small engine. It'll only come into play at very high flow rates (large cubic inch engines with lots of air flow / velocity or... smaller engines at very high RPM's) How often is your mustang "running" at 7,000 - 8,000 RPM?

The down side to milling the air horn is that it prevents the choke horn from "straightening out" the incoming air charge. In other words, it acts like a wind tunnel, putting a cleaner, straighter, less turbulant flow pulling through the boosters, (and on 3310's, through the vacuum secondary siphon hole). It makes the carb more responsive at lower RPM's by having it on there.

You have to keep in mind, just because you see things like this done on race cars doesn't mean it works on street cars. Even in some race applications, milling the air horn is a no-no if the carb is out in the open, or isn't directly under an air scoop. This is because at high speeds, air passes across the top of the carb and can actually pull air back out, starving the carb. You need air to get down inside the carb, not skate across the top, which causes a siphoning effect on the carb, which can actually starve the carb for air.

Just remember, it ain't Holley's first day. ALL of their street carbs have air horns for a reason. Their race series carbs (such as thei HP series) don't have air horns because they figure this kind of stuff is taken into consideration.

Removing the choke assembly is a popular task, and we do it on pretty much every Holley we use. Holley's don't really need a choke. Pump the gas 2 times when it's cold and fire it up. Just keep your foot in it for abouit 1 minute, until the engine begins to warm up, and you're good to go. Using a choke on a Holley "can" making it easier to flood the engine.

Hope that helped

Any kind of choke plate position other than wide open will instantly flood my engine. I yanked the plate and let the choke control only the high idle.

My milled choke experiment was one of those "ignorant kid with a car" things I did, back when I thought modding for 1 extra horsepower at 6000RPM on a street car was actually worth my time.

I did a 650 DP using the hacksaw and diegrinder method as well. If you take the time and be carefull, there's nothing to getting a factory finish in doing this operation.

D.Hearne said:

I did a 650 DP using the hacksaw and diegrinder method as well. If you take the time and be carefull, there's nothing to getting a factory finish in doing this operation.

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i've done this also except i normally leave the remnants of the choke horn in place instead of smoothing and contouring it into the venturis, it will have absolutely no effect on flow if it is a street only carb.

has anyone seen holley's new line of all aluminum carbs? i guess they started making these in response to the Proform and Quick Fuel aluminum carbs.

Quick little trick I did after mill the horn and blending with a die grinder was to treat the exposed alum with gun blue. It stained the metal close to the original anodized finish and looked very professional